Oil-in-water emulsion compositions are widely used as bases in skin-care preparations for external use, such as skin-care cosmetics or the like, that are applied directly to the skin, because they provide a crisp and fresh sensation when applied to the skin. In particular, skin care and body care have come to include everyday protection of the skin from UV rays, and the importance of using oil-in-water emulsion compositions as bases in such UV-care cosmetics is also increasing.
On the other hand, while metal oxide powders are known to have the function of UV-scattering agents that protect the skin from UV rays, aside therefrom, they also have the functions of improving the feeling in use during application, and concealing coloration and blotches, freckles and the like on the skin, so they have conventionally been widely added to various types of cosmetics. In recent years, metal oxides having hydrophobically treated surfaces have been used for the purposes of improving water resistance, cosmetics durability and cosmetic dispersibility.
When blending a hydrophobically treated metal oxide powder such as hydrophobic fine-particle titanium oxide or hydrophobic fine-particle zinc oxide into an oil-in-water emulsion composition, it is usually dispersed in a volatile oil using a low-HLB dispersing agent or special dispersing equipment such as a wet bead mill with strong pulverization energy to be emulsified. However, in order to mix a lot of powder in order to obtain sufficient UV ray protection capability, a large quantity of oil becomes necessary for dispersing the powder, as a result of which the feeling in use can become oily and the freshness that is inherent in oil-in-water emulsion compositions can be lost. Additionally, if the dispersion medium of the powder evaporates after application to the skin, the powder can re-aggregate in the formed film and cause the lost transparency.
In sunscreen cosmetics, it is common to make combined use of UV-absorbing agents and UV-scattering agents such as metal oxide powders in order to achieve UV protection effects against a wide wavelength range from the UVA range to the UVB range, but most UV-absorbing agents are non-volatile oils, so the total oil content, when also adding the oils used as the powder dispersion medium, becomes high, and the relative water content decreases, making it difficult to achieve a fresh feeling in use.
For example, Patent Document 1 describes an oil-in-water emulsified sunscreen cosmetic wherein, in order to stably disperse a hydrophobically treated zinc oxide without using a special dispersion apparatus, a specific compound (octyltriethoxysilane or dimethylpolysiloxane) is used for the hydrophobic treatment of the zinc oxide, and this is dispersed in a specific oil (liquid higher fatty acid) using a specific dispersing agent (a carboxyl group-containing silicone or sugar ester). However, a large amount of volatile oil is required in order to stably disperse hydrophobically treated zinc oxide, as a result of which the total oil content increases and a fresh feeling in use is difficult to achieve.
Additionally, Patent Document 2 describes that, in order to stably blend hydrophobically treated powder particles into an oil-in-water emulsion composition that uses an ionic water-soluble polymer compound as a thickener, hydrophobic powder particles are used as the powder particles, and furthermore, ion elution is prevented by mixing a specific polyether-modified silicone into the oil phase.
In this composition, the polyether-modified silicone acts as a gelling agent for gelling the oil phase which includes a silicone oil and hydrophobic powder particles, and prevents ion elution from the hydrophobic powder particles in the oil phase, thereby stabilizing the thickening due to the ionic polymers in the water phase. However, since the dispersion of powder in an oil phase is conventionally performed by mechanical force and a large amount of silicone oil must be added as a dispersion medium in order to sufficiently disperse the hydrophobic powder particles, as with Patent Document 1, the total oil content increases and it is difficult to achieve a fresh feeling, in use.
On the other hand, aside from the above-mentioned Patent Document 2, there have been other attempts to achieve stabilization of ail-in-water emulsion compositions using polyether-modified silicones (Patent Documents 3 to 5). Patent Document 3 describes that a non-sticky, fresh sensation in use, stability and a high SPF can be achieved by mixing a combination of polyether-modified silicone, a predetermined amount of an extender pigment, and a UV-absorbing agent having absorption capability in the UV-A region. Patent Document 4 describes that excellent water resistance can be obtained after application to the skin, by forming an oil-in-water emulsion using a polyether-modified silicone having an HLB (Si) of 5 to 10 as a surfactant, and further adding a predetermined amount of ethanol, a hydrophilic thickener and a polyol. Patent Document 5 describes that, by blending an aqueous dispersion of an oil-soluble UV-absorbing agent into the water phase in a stable system similar to that of Patent Document 4, the UV protection effect was increased over the case of blending a UV-absorbing agent into an oil phase.
However, none of Patent Documents 3 to 5 teaches a method of blending a high content of a hydrophobically treated metal oxide. On the contrary, Patent Document 3 describes that the emulsion stability becomes worse when adding hydrophobically treated talc as compared with the case where talc that has not been hydrophobically treated is added (Comparative Example 4), and Patent Document 5 describes that the feeling in use is inferior when fine-particle titanium oxide is added (Comparative Example 2).